1. Field of the Invention
The present invention relates to switching circuits generally and more particularly to a high-voltage switching circuit employing relatively inexpensive transistors.
2. Description of the Related Art
It is not uncommon for transistors to be employed in relatively high-voltage switching circuits. However, in most case, the voltages involved subject the transistors to relatively high stresses. Heretofore, to withstand the stresses, it has usually been necessary to employ relatively expensive transistors.
In some prior-art-type switching circuits, two (or more) transistors are configured with their channels connected in series, to reduce the level of stress each of the series-connected transistors must withstand. However, the problem has been one of ensuring that the voltage developed across the series-connected transistors is divided relatively equally. In one prior-art-type configuration, the gate of the "upper" transistor is connected to a fixed low-level potential and the gate of the "lower" transistor is connected to receive a switching signal. Unfortunately, in such a configuration, the "upper" transistor is usually subjected to a disproportionally high level of stress.
One application for high-voltage switching circuits is in electronic ballast circuits employed to drive fluorescent lamps. A "Single-Ended Ballast Circuit," is disclosed in U.S. Pat. No. 5,028,846. Briefly, the above-mentioned circuit employs a DC power supply, a free-running oscillator, a switching circuit (transistor), and a current-limiting (ballasting) network, all configured to generate a high-frequency AC power source on a line. Specifically, the transistor is configured as a switch to be responsive to a signal generated by the oscillator and to be operative to periodically couple the line to a circuit common. For this purpose, the transistor is connected with its gate connected to the output of the oscillator, with a drain connected to the line, and a source connected to the circuit common. The current-limiting (ballasting) network includes a first capacitor connected between the line and the circuit common. In addition, the network includes a transformer (inductive means) having a transformer primary winding connected between a DC power supply potential and the line, and a transformer secondary winding. Finally, the network includes an inductor and two capacitors all connected in series across the transformer secondary winding. Two fluorescent lamps are connected in series across one capacitor. The network is operative to provide an impedance transformation to couple the fluorescent lamps to the high-frequency AC power source developed on the line. Also, the network develops the desired open circuit output voltage for starting the fluorescent lamps. In addition, the network provides the desired source impedance, as seen by the lamps. Also, the network establishes the operating Q for the desired output waveform. Further, the network provides the desired load impedance and phase angle as seen by the transistor for both the operating and open circuit conditions. Finally, the network is operative to provide an inductive power-supply feed for the transistor. Unfortunately, in this ballast circuit, voltage stresses are generated in the switching circuit transistor which require the use of a relatively expensive transistor, especially for AC line source voltages of 240 volts and higher.